This invention relates generally to a rotary valve, and more particularly, to a rotary valve for pressure swing absorption systems.
Rotary valves are widely used in the process industries for directing fluids from one or more process sources to one or more process destinations in repeatable cyclic process steps. These valves, also called rotary sequencing valves, are used in cyclic or repeatable processes, such as gas separation by pressure or temperature swing adsorption, liquid separation by concentration swing adsorption, gas or liquid chromatography, regenerative catalytic processes, pneumatic or hydraulic sequential control systems, and other cyclic processes.
A widely used type of rotary valve has a planar circular configuration in which a flat, ported rotor rotates coaxially on a flat, ported stator such that ports in the stator and rotor are either aligned or blocked in a predetermined cyclic sequence. Sealing typically is provided by direct contact mating of the flat rotor face over the flat stator face. A high degree of precision is required in the fabrication of these flat surfaces to prevent excessive leakage at the mating surfaces. Rigid materials such as metals, ceramics, and/or carbon are typically are used for these rotors and stators, but wear of the parts or distortions caused by temperature differentials may cause changes in the shape of the surfaces, thereby allowing leakage across the seal formed between the surfaces.
Rotary valves with a flat rotating circular seal configuration are particularly useful in pressure swing adsorption (PSA) systems utilizing multiple parallel adsorber beds operating in overlapping cyclic steps that include feed, pressure equalization, depressurization, purge, and repressurization steps. In a typical application, a stator having multiple ports is used to connect feed gas and waste gas lines with the feed ends of a plurality of adsorber beds and also to connect the product ends of the plurality of the beds to provide pressure equalization, purge, and other bed-to-bed transfer steps. A rotor having multiple ports sealably rotates on the stator such that the openings on the stator face register sequentially with openings in the rotor face as the rotor rotates to direct gas flow for the desired PSA process cycle steps.
In a typical PSA cycle, the internal passages of the rotary valve are at different pressures as the PSA cycle proceeds. When the PSA cycle includes process steps at positive pressure and under vacuum, leakage driven by the pressure differentials between the valve ports connected to the feed and product ends of the beds may lead to various operating problems if leaks occur between these ports.
Rotary sequencing valves, in which a flat, ported rotor rotates coaxially on a flat, ported stator such that ports in the stator and rotor are aligned or blocked in a predetermined cyclic sequence, are used for directing fluids in cyclic processes having a number of repeatable steps. In U.S. patent application Ser. No. 11/197,859 (hereinafter referred to as the '859 application) filed May, 8, 2005, the disclosure of which is incorporated by reference in its entirety, a dual rotor/stator rotary valve system is disclosed that uses a single axially aligned spring to assist in mating a rotor surface against a stator surface to assist in sealing the rotor and stator surfaces against one another and prevent leakage between stator and rotor ports. The rotor ports are located at different circumferential positions on the rotor faces and operate at different pressures.
During operation of the prior art rotary valve disclosed in the '859 application, the difference in the port pressures results in a non-axial force across the rotor and stator mating face. When high operating pressures are required, large spring forces may be required to seal the rotors against the stators and prevent leakage. The amount of force necessary to turn the rotors will be directly related to the amount of force the spring compresses the rotors against the stators. If high spring forces are required to prevent leakage between the rotor and stator, large forces will be required to turn the rotors. These large forces increase rotor wear, require larger rotor motors, and increase rotor bearing wear.
The general arrangement of an exploded view of prior art rotary valve 1, such as found in the '859 application, is shown in FIG. 1. In actual operation, the components of the valve 1 are in contact with one another. As can be seen in FIG. 1, the prior art rotary valve 1 includes a feed stator 10, a feed rotor 20, a product rotor 30, a product stator 40, and a compression spring 50. In this exemplary prior art embodiment, the feed rotor 20 and the product rotor 30 are contained within a housing formed by the feed stator 10 and the product stator 40 as shown in FIG. 1.
In a pressure swing adsorption (PSA) process, adsorber beds (not shown) are connected to the ports 11a, 11b, 11c, 11d of the feed stator 10 and the ports 41a, 41b, 41c, 41d of the product stator 40. The feed end of the beds (not shown) are typically connected to ports 11a, 11b, 11c, 11d of the feed stator 10, and the product ends of the beds (not shown) are typically connected to the corresponding ports 41a, 41b, 41c, 41d of the product stator 40.
As can be seen in FIG. 1, the feed rotor 20 and the product rotor 30 are configured to mate and interlock. The compression spring 50 is disposed between the feed rotor 20 and the product rotor 30. The compression spring 50 presses the feed rotor 20 against the feed stator 10 to seal the feed rotor 20 against the feed stator 10. The compression spring 50 similarly presses the product rotor 30 against the product stator 40 to seal the product rotor 30 against the product stator 40.
The known valve 1 further includes a drive shaft 60 capable of rotating the feed rotor 20 and the product rotor 30. Drive shaft 60 includes a positive drive end 62 that is configured to engage a mating feature (not shown) in the feed rotor 20 in such a manner that when drive shaft 60 is rotated, feed rotor 20 and product rotor 30 are likewise rotated about an axis perpendicular to the rotor face, and slots within the feed rotor 20 and the product rotor 30 are aligned with ports in the feed stator 10 and the product stator 40, respectively, to select a predetermined connection of process lines.
The known rotary valve 1 includes various fluid ports and passages, the function of which are more fully disclosed in the '859 application. The operation of a specific cyclic process, such as PSA, need not be completely explained herein to understand the valve operation, and would be understood by one of ordinary skill in the art. In general, process operations include altering the rotated position of the feed rotor 20 and the product rotor 30 of the known valve 1 to allow for select fluid steams to be cycled. A general description of the operation of the prior art rotary valve 1 will now be provided.
As the feed rotor 20 and the product rotor 30 are rotated to predetermined positions, ports in the rotor faces are aligned with ports in their respective stators, allowing flow to and from the valve 1 through a predetermined connective path. In such a manner, fluid may flow between the beds connected to the feed stator 10 and product stator 40 as necessary for equalization, purge, or other cyclic process steps.
In a PSA process, the pressure in the beds alternates between high pressures and low pressures where adsorption and desorption take place, respectively. During process operations, the pressures within each slot exert a force on the feed rotor 20 and the product rotor 30, urging them away from the feed stator 10 and the product stator 40, respectively. For this reason, compression spring 50 is required to hold the feed rotor 20 against the feed stator 10 and the product rotor 30 against the product stator 40 to prevent leakage. Because the spring force and the pressure forces within the slots are not symmetric about the center or rotation of the rotors, the resultant force on the rotors is not located at the center of both the feed rotor 20 and the product rotor 30. This asymmetric force load results in a need for an increased spring force necessary to maintain rotor/stator contact, as well as increased torque required to actuate the valve and turn the shaft 60.
FIGS. 2A, 2B and 2C show simplified views of the forces acting on a rotor 200 of a rotary valve during a typical PSA cyclic process. The center axis of rotation of the rotor 200 is indicated by the vertical dashed line A′. The spring force F1 is the force exerted by a spring (not shown) on the rotor 200 as it pushes the rotor 200 against a stator (not shown). The pressure force F2 is the resultant force from the pressures in the various ports. The reaction force F3 is the difference between the spring force F1 and the pressure force F2. F3 is also the contact force between the rotor 200 and the stator (not shown). The reaction force F3 is not located at a single point. The reaction force F3 is distributed along the rotor 200 in some manner, which may be very complicated, depending on the flatness of the mating faces, the magnitude of the force, and the slight deformation of the rotor 200 caused by the applied loads. However, for simplicity, this distributed force may be resolved into the single resultant reaction force F3. There must always be a non-zero reaction force F3 if the rotor 200 and stator (not shown) are to remain in contact. If the reaction force F3 is zero or less, then the pressure force will begin to separate the rotor 200 from a stator, and leaks will occur between various ports of the rotor and stator (not shown). It is the reaction force F3 that is responsible for friction torque between the rotor and stator, through both its magnitude and location, and determines the amount of torque necessary to turn the rotor 200.
FIG. 2A shows the forces acting on the rotor 200 if the pressure force F2 is located at the center of the rotor 200. In this example, all of the forces are collinear, and the reaction force F3 is the difference between the spring force F1 and the pressure force F2. This result only happens if the pressure force F2 is a resultant of a symmetrically balanced pressure forces around the rotor 200. This symmetrical distribution does not exist since the pressures of the various slots of the rotor 200 will not result in a net pressure force acting at the center of the rotor 200 during typical PSA process operations.
FIG. 2B shows the distribution of forces acting on the rotor 200 when the pressure force is not located at the center of the rotor 200, as would occur during typical PSA process operations. Similar to the discussion of FIG. 2A above, the reaction force F3 is the difference between the spring force F1 and the pressure force F2, but now, in order to maintain equilibrium on the rotor 200 to keep the moments in balance, the reaction force F3 must also shift from the center of the rotor 200 to a radial position away from the rotor center. The location and magnitude of the reaction force F3 depends on the locations and magnitudes of the spring force F1 and the pressure force F2. Also, since the spring force F1 must be equal to the sum of the pressure force F2 and the reaction force F3, the spring force F1 must always be greater than the pressure force F2 whenever the spring force F1 and pressure force F2 are not collinear.
When the spring force F1 and pressure force F2 are not collinear, they produce a bending moment 210 in the rotor 200 as indicated by the dashed line in FIG. 2B. The bending moment 210 may deform the rotor 200 if the forces are of sufficient magnitude for a particular rotor material and thickness. In some applications, this deformation may be maintained small enough to prevent leakage by making the rotor more rigid, either through the use of more rigid materials, or by increasing the rotor thickness. For larger rotors, this may become impractical. Additionally, the eccentricity of F3 will increase the torque required to turn the rotor 200 during process operations.
Thus, it would be desirable to relocate the spring force F1 as shown in FIG. 2C to a predetermined radial distance from the center axis of rotation A′, opposite the pressure force F2, to eliminate the bending moment in the rotor 200. The reaction force F3 would then be acting in the same position as the pressure force F2. This rearrangement results in the lowest required spring force F1 and reaction force F3 and a lower torque necessary to turn the rotor 200.
Even in applications when the bending moment and deflection are not of significant concern, the torque required to turn the motor and rotate the rotors may be a significant concern, especially when high pressures are present in the rotor ports. Usually, it is desirable to keep this torque to a minimum, since reducing torque reduces the size and/or increases the life of the motor and gear drive necessary to turn the rotor.
Thus, there is a need for a rotary valve that is capable of operating without leakage and having a reduced torque required to rotate the valve rotor.
This invention provides for a rotary valve capable of operating under such conditions without substantial leakage and with minimum torque required to turn the rotor. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings that illustrate, by way of example, the principles of the invention.